Defensins are small, basic, cysteine-rich proteins that exhibit broad antipathogenic activity through the formation of multimeric pores in outer or inner biological membranes. The multimeric pores lead to membrane disruption and depolarization. Defensins have a wide phylogenetic distribution, having been found in insects, mammals, and plants.
Although plant defensins have only been identified recently and are not as well characterized as their mammalian and insect counterparts, several lines of observation suggest the importance of defensins in mediating host resistance to pathogen attack. Plant defensins have been shown to induce a rapid K+ efflux and Ca2+ influx in fungal hyphae as well as alkalinization of the incubation medium. The operating mechanism however appears not to involve direct defensin-membrane interactions, but rather a different, possibly receptor-mediated, event (Thevissen, K., et al. (1996) J. Biol. Chem. 271:15018-15025). Defensins have also been shown to accumulate systemically upon challenge by fungal pathogens (Manners, J. M., et al. (1998) Plant Mol. Biol. 38:1071-1080; Terras, F. R., et al. (1998) Planta 206:117-124; Terras, F. R., et al. (1995) Plant Cell 7:573-588). Furthermore, transgenic tobacco that constitutively expressed a radish defensin was found to have improved resistance to infection by a fungal pathogen (Terras, F. R et al., (1995) Plant Cell 7:573-588).
Defensins have been shown to be induced by artificial drought (Maitra, N. and Cushman, J. C. (1998) Plant Physiol. 118:1536) and salt stress (Yamada, S., et al. (1997) Plant Physiol. 115:314) suggesting that these proteins may play a more general role in stress tolerance, one that is not restricted to pathogen attack.
Defensin molecules may be used in transgenic plants in order to produce plants with increased resistance to pathogens such as fungi, viruses, bacteria, nematodes, and insects. Thus, the present invention solves needs for the enhancement of a plant's defensive response via a molecularly based mechanism that can be quickly incorporated into commercial crops.